The Chemical Evolution of Ground Water in the Milk River Aquifer, Canada

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Abstract
Well‐defined trends are observed in the ion composition of ground water from the Milk River aquifer. Ground water from the area of the subcrop has higher concentrations of Na+, SO42‐, Ca2+, and Mg2+ than immediately downgradient. Away from the area of subcrop, Na+, Cl‐, HCO3‐ + CO32‐, and CH4 concentrations increase systematically with increased residence time, pH decreases, and Mg2+ and Ca2+ concentrations are typically low (less than 0.1 mmol/1). Geologic changes play an important role in producing these chemical patterns. The first major geologic change was the erosion of the overlying confining beds in the recharge area about 5 × 105 years ago, enabling meteoric water with low concentrations of Na+ and Cl‐ to enter the aquifer and displace preexisting water. The second major change was the deposition of glacial till in the area of the Milk River about 30,000 to 40,000 years ago. Water recharging through the till to the aquifer developed characteristically high concentrations of Na+, SO42‐, Ca2+, and Mg2+. Downgradient of the area of subcrop, the trends in Na+ and Cl‐ are controlled by diffusion from the underlying confining shale. Analyses of ground‐water and gas samples for sulfate reducers and ground‐water and gas samples for sulfide and H2S, respectively, suggest that SO42‐ reduction is not a major process. Geochemical modeling suggests that CO2 gas is added to the ground water with increased residence time in the aquifer. The increase in CO2, CH4, and dissolved inorganic carbon can be attributed to methane fermentation. Geochemical modeling suggests that cation exchange plays a minor role in the chemical evolution of the ground waters.